We have found that many abused drugs increase heart rate (HR) and decrease cardiac vagal tone index (V). Traditional views of the limited effect of the opiates on the cardiovascular system would suggest that the opiates represent an exception to this observation. However, this may be due to recordings taken after compensatory mechanisms have stabilized cardiovascular parameters. Therefore, we recorded HR and V continuously before, and for 30 min after i.v. morphine (15 mg) injection to measure initial cardiovascular responses. V is a well-validated measure of vagal inhibition of the heart based on time series analysis of HR variability entrained with respiration (i.e., respiratory sinus arhythmia). The subjects were 9 opiate abusers who were research volunteers living on a residential ward. Intravenous morphine produced immediate HR increases (p<.01) of +9 bpm, and V decreases (p<.05) of -1.4 log units. These responses gradually returned toward baseline during the 30 min after injection. These effects were in the same direction, but of larger magnitude than our previous results with i.m. morphine (20 mg). Therefore, morphine may not be an exception to the pattern of cardiovascular responding to various abused drugs when considering initial responses before compensatory mechanisms develop. Moreover, these results indicate that the HR response to morphine has a substantial parasympathetic component.